Obey God and do the works of the law by gospel principles and means. This is the rare and excellent art of godliness, in which every Christian should be a skilled expert. Many people labor for years trying to live a godly life. However, they give it up in shame and confusion because they never understood this holy art. They never tried to attain true godliness in a proper gospel way.
Walter Marshall, The Gospel Mystery of Sanctification: Growing in Holiness by Living in Union with Christ (Wipf & Stock 2005), 167-68; emphasis original
I'm grateful to Dan Orr for pointing me to this book.